I. Display Quality Glass Sheets
Historically, display quality glass sheets have been commercially produced using the float process or the fusion overflow downdraw process (fusion process). In each case, the process involves four basic steps: melting batch materials, fining (refining) the molten glass to remove gaseous inclusions, conditioning the refined glass to prepare it for forming, and forming, which in the case of the float process involves the use of a molten tin bath, while for the fusion process, involves the use of a forming structure, e.g., a zircon isopipe. In each case, the forming step produces a ribbon of glass which is separated into individual glass sheets. The sheets are inspected and those that meet the customer's requirements are finished and delivered. The sheets that fail to pass inspection are normally crushed into cullet and remelted with new raw materials.
The goal for both the float and fusion processes is to produce glass sheets having low levels of defects, i.e., low levels of gaseous and solid defects. More particularly, the goal is to achieve a low level of defects for the glass sheets as manufactured so as to reduce the number of sheets that are rejected by the inspection process. The economics of the process and thus the cost of the glass sheets are highly dependent on the reject level.
Gaseous defects are introduced into the molten glass during the melting process, as well as downstream through such mechanisms as hydrogen permeation (see Dorfeld et al., U.S. Pat. No. 5,785,726). Solid defects can originate from the batch materials, as well as from the refractories and/or heat-resistant metals that come into contact with the molten glass as it moves through the process. Wear of the glass-engaging surfaces of the furnace used to melt the batch materials is one of the primary sources of solid defects. A common material for the walls of a melting furnace is zirconia, e.g., electrocast zirconia, and thus the formation of zirconia-containing solid defects has been and continues to be a challenging problem in the manufacture of display quality glass sheets.
As the demand for products employing display quality glass sheets has increased, manufacturers of such products have sought glass sheets of ever larger dimensions in order to achieve economies of scale. For example, the current sheets supplied to manufacturers of flat panel displays are known as Gen 10 sheets and have dimensions of 3200 mm×3000 mm×0.7 mm. From the point of view of glass manufacturers, the production of larger display quality glass sheets means that more glass has to be moved through the manufacturing process per unit time. However, this increase in production rate cannot be achieved through compromises in the quality of the sheets supplied to the customer. Indeed, as the resolution of display products has and continues to increase, the quality of the glass sheets used in such products has and must continue to improve. In terms of rejects, larger sheets make reducing the levels of solid and gaseous defects even more important because each rejected sheet represents more glass that was produced but not supplied to a customer. The higher quality standards demanded by customers only exacerbates this problem.
One of the key limiting steps in the production of high quality glass sheets is glass melting and the subsequent fining (refining) of the molten glass to remove gaseous inclusions. In the past, melting has been accomplished through a combination of burning fossil fuels (e.g., methane) and direct electrical heating (Joule heating). The Joule heating has been performed using tin oxide electrodes. These electrodes have set an upper limit on the production rate of display quality glass sheets. In particular, as illustrated in FIGS. 6-8 and discussed below, for melters whose glass-engaging surfaces are composed of zirconia, it has been found that the rate of wear of the walls of the melter increases substantially as the current through the tin oxide electrodes is increased to accommodate higher production rates. This increased wear translates into increased concentrations of dissolved zirconia and increased levels of zirconia-containing solid defects in the finished glass sheets. In addition to the wear problem, when electricity passes through tin oxide electrodes it generates bubbles at the interface between the electrode and the molten glass. These bubbles represent an additional load on the finer (refiner) used to clarify the molten glass.
In the glass industry, melting effectiveness is often reported in units of square feet/ton/day, where the square feet is the footprint of the melter and the tons/day is the flow rate through the melter. For any designated pull rate (flow rate), the smaller the square feet/ton/day number the better since it means that less square footage will be required in a manufacturing plant to achieve the desired output. For ease of reference, melting effectiveness defined in this way will be referred to herein as the furnace's “QR-value” given by the formula:QR=Afurnace/R  (3)where Afurnace is the horizontal cross-sectional area of the molten glass in the melting furnace in square feet and R is the rate at which molten glass leaves the furnace and enters the finer in tons of glass per day.
As a consequence of the limitations imposed by tin oxide electrodes, in practice, the maximum flow rates and associated QR-values for commercial melters using such electrodes to melt display quality glasses have been 1,900 pounds/hour at a QR-value in the range of 6-7 square feet/ton/day. Above this flow rate, defect levels rise rapidly to unacceptable levels. Although such a flow rate and associated QR-value is adequate for many applications, melters that are capable of operating at higher flow rates, e.g., at flow rates above 2,000 pounds/hour, without substantial increases in QR-values are desirable to enable the industry to meet the ever growing demand for large, display quality, glass sheets. Achieving such higher flow rates with QR-values below 6.0 square feet/ton/day is even more desirable.
II. Melting Furnaces Employing Zirconia-Containing Glass-Engaging Surfaces
Japanese Patent Publication No. P2010-168279A, which is entitled “Method for Manufacturing Alkali-Free Glass” and is assigned to the Nippon Electric Glass Co., Ltd. (hereinafter the '279 application), discusses the problem of elution of zirconia from melting furnaces whose walls are made of zirconia refractories. As described in paragraph [0022] of this reference, “it was discovered that when an alkali-free [display] glass . . . is melted with manufacturing equipment that uses high zirconia-based refractories, a ZrO2 component elutes from the refractories and the ZrO2 concentration in the glass intensifies, and devitrification occurs very easily . . . . ”
The '279 application seeks to address this problem by constructing the “supply passage” of its glass making system out of platinum or a platinum alloy, where in the terminology of the '279 application, “the ‘supply passage’ means all of the equipment provided between the furnace and the molding device.” ('279 application at paragraph [0061].) As explained in the '279 application, “the greater the portion [of the supply passage] formed with platinum or platinum alloy, the better, and ideally the entire surface that is in contact with the glass is formed of platinum or platinum alloy.”
Importantly, the '279 application contains no recognition of the discovery of the present application that by using molybdenum electrodes instead of the tin oxide electrodes normally used in the melting of display quality glasses, the specific wear rate of a melting furnaces glass-engaging surfaces composed of zirconia can be reduced by more than 50%. Rather, the '279 application treats tin oxide, molybdenum, and platinum electrodes as fungible and in selecting an electrode, considers only electrode wear and contamination of the glass by the elution of the electrode material, not the effects of the choice of electrode on the wear rate of the walls of a furnace made from a zirconium-containing material. See paragraph [0060] of the '279 application (“There is no particular restriction to the electrode material; the material can be selected appropriately by considering the life of the electrode, the degree of corrosion, and the like.”).
Moreover, in its Application Examples, the '279 application uses tin oxide electrodes. See paragraph [0090] of the '279 application (“direct electrical heating by means of an SnO2 electrode was performed”). In using tin oxide electrodes, the '279 application is following the conventional wisdom that for highest quality glasses, such as the borosilicate glasses used for display applications, tin oxide electrodes should be used. See Argent, R. D., “Modern Trends in Electrode Utilization,” IEEE Transactions on Industry Applications, January/February 1990, 26:175, 180 (“[B]orosilicate-type glasses are among a group of glasses that demand the highest quality requirements. Seed and blister are usually not tolerated, and as such, the tin oxide electrode has become commonplace when manufacturing these glasses.”)
In its Application Example 2, the '279 application achieves a ZrO2 concentration in its finished glass of 0.2 weight percent. See Table 3 of the '279 application. This concentration is substantially higher than the concentrations achieved using the technology of the present disclosure. In particular, zirconia concentrations in the finished glass that are at least 50% lower, i.e., levels less than or equal to 0.1 weight percent, e.g., levels less than or equal to 0.05 weight percent, are readily achieved using the present technology.
U.S. Patent Application Publication No. US 2011/0120191, which is entitled “Fusion Processes for Producing Sheet Glass” and is assigned to Corning Incorporated (hereinafter the '191 application), also relates to the problem of zirconia eluting from melting furnaces made from zirconia-containing refractories. The approach of the '191 application is to control the temperature distribution of the glass making system so that zirconia that has entered the glass does not crystallize out of solution and form zirconia-containing solid defects. As with the '279 application, the '191 application does not address the source of the zirconia in the molten glass, i.e., the wear rate of the zirconia furnaces glass-engaging surfaces when melting display quality glasses, and thus does not provide a method or apparatus for reducing the level of zirconia-containing solid defects in display glasses.
A low wear rate and thus a low concentration of zirconia and a low level of zirconia-containing solid defects in the finished glass is just one criterion for a successful melting furnace for display quality glass sheets. Other criteria include the ability to achieve high flow rates, ease of fining, compatibility with the agents used to fine (refine) “green” glasses (i.e., glasses that do not contain arsenic or antimony), and low levels of contamination of the display quality glass by the electrode material. As demonstrated below, in addition to reducing zirconia concentrations and defect levels, the melting furnaces disclosed herein satisfy these and other criteria for an effective melting furnace for display glasses.